HM-SAGITTAE - THE SHOCKED AND PHOTOIONIZED INTERACTION ZONE OF 2 COLLIDING STELLAR WINDS

We present a model for the interaction zone of the colliding winds from the two stellar components of the symbiotic binary system HM Sge. The plasma is ionized and excited by shock, as well as by radiation. Intensities of all emission lines, of high, intermediate, and low ionization levels, computed by the model, provide a good fit to the corresponding observed values. The model is also used to describe successfully the evolution, on a time-scale of a few years, of the observed intensities of the emission lines and of the continuum, in the visible, UV, radio and X-ray regions of the spectrum, following the outburst of the star in 1975. All the observed changes are consistently interpreted as reflecting the weakening of the shock, presumably caused by a gradual decrease of the stellar wind from the outburst region, and the steady increase in the colour temperature of the hot component in the system. We also find that the shocked gas is predominantly composed of the material of the red giant.